The present invention relates generally to gas turbine engines, and, more specifically, to turbine rotor blades therein.
In a gas turbine engine, air is pressurized in a compressor and mixed with fuel in a combustor for generating hot combustion gases. Energy is extracted from the gases in various turbine stages in a high pressure turbine (HPT) and a low pressure turbine (LPT).
The HPT includes one or more stages of rotor blades extending from a supporting rotor disk which drive the corresponding rotor blades of the compressor. And, the LPT typically includes several rotor stages of blades extending from corresponding disks which drive an upstream fan in an aircraft engine application, or drive an external drive shaft for marine and industrial applications.
An aircraft engine is used to power an aircraft in flight in various locations around the world. The engine is therefore subject to ingestion of dust and other pollutants along with the intake air. Dust is a generic term for the various particulates or foreign body contamination found in the ambient air through which the aircraft flies, and which may be harmful to the longevity of the engine.
For example, the ambient air may be polluted from automotive vehicles and industrial plants typically concentrated around major cities, which also typically have regional airports. Airports in cold regions typically use chemical deicers in the winter months for de-icing aircraft prior to flight. And, the eruption of volcanoes additionally discharges substantial contamination into the atmosphere.
Accordingly, the atmosphere found around the world contains various pollutants which can adversely affect the life of a gas turbine engine. The terms dust is used herein as a generic term for the various forms of foreign body matter which may be found in the air that may adversely affect engine life. Typical dust is in the form of fine particles of foreign matter like sulfur, sodium, potassium, and magnesium for typical examples found in various aircraft routes around the world.
Dust can be a problem in gas turbine engines because it is ingested with the ambient air and is subject to the hot combustion process, as well as remains in the air bled from the compressor in typical use for cooling hot engine components.
The HPT is subject to the hottest combustion gases and is therefore configured with various cooling circuits that circulate pressurized cooling air from the compressor. The cooling circuits include small channels and typical film cooling holes through which the pressurized air must pass.
Since the film cooling holes are relatively small, plugging thereof by dust during operation would severely shorten the useful life of the component. In a typical turbine rotor blade, for example, the tip thereof may include relatively large dust escape holes to provide a convenient location for the discharge of undesirable dust and minimize its accumulation inside the hollow blade.
However, recent experience in use of an exemplary gas turbine engine enjoying years of successful commercial public use in this country and abroad has uncovered a new problem of dust accumulation under the platform of turbine blades.
A turbine rotor blade typically includes a hollow airfoil extending radially outwardly from a platform which defines the radially inner flowpath boundary. The platform in turn is joined to a plain shank that terminates in a dovetail for mounting the blade in a corresponding slot in the perimeter of the supporting rotor disk.
Since energy is extracted from the combustion gases as they flow downstream between the turbine blades in each stage or row, there is a corresponding pressure drop between the leading and trailing edges of the blades. Pressurized cooling air is bled from the compressor and channeled to the blades through inlets at the base of the dovetails thereof. The shank regions of the blades are typically provided with pressurized purge air to prevent the ingestion of combustion gases therein.
Accordingly, dust-laden air is not only channeled through the individual turbine rotor blades, but is also channeled outside the blades under the corresponding platforms, and is therefore subject to undesirable accumulation in any local surface discontinuity or pocket formed by the various portions of the turbine blade below the platform.
For example, the platform joins the shank at corresponding fillets which is one site for accumulation of dust. The platform also joins the shank at forward and aft angel wing seals that define additional internal corners and fillets in which dust may accumulate. And, turbine rotor blade may also include dampers which are trapped in corresponding pockets between adjacent blades, with these pockets also providing additional sites for undesirable dust accumulation.
Furthermore, the turbine blade extends radially outwardly from the rotor disk and is subject to considerable centrifugal force during operation. The centrifugal force also acts on the dust particles in the cooling air driving these particles into stagnant fillets or corners or pockets where they may accumulate.
The recent experience described above has shown that the accumulation of undesirable dust can lead to corrosion of the superalloy metal materials that form the turbine blades, which further shortens the useful life of the turbine blades in the hostile environment of the gas turbine engine.
Accordingly, it is desired to provide a turbine rotor blade with a dust proof or resistant platform for minimizing the accumulation of undesirable dust therein over the life thereof.